My markers in the movie clip editor keep "moving" around - what gives?
The issue might be caused by inconsistent decoding of your particular video codec by Blender, resulting in variation in the frame sequence you move through on the timeline. My suggestion would be that you convert the video to a discrete image format, like a PNG or JPEG sequence. A typical way to convert video to an image sequence:

ffmpeg -i "MyVideo.mov" -an -f image2 "MyVideo%06d.png"